Conventional speech coding methods are generally based on single-channel speech signals. An example is the speech coding used in a connection between a regular telephone and a cellular telephone. Speech coding is used on the radio link to reduce bandwidth usage on the frequency limited air-interface. Well known examples of speech coding are PCM (Pulse Code Modulation), ADPCM (Adaptive Differential Pulse Code Modulation), sub-band coding, transform coding, LPC (Linear Predictive Coding) vocoding, and hybrid coding, such as CELP (Code-Excited Linear Predictive) coding [1-2].
In an environment where the audio/voice communication uses more than one input signal, for example a computer workstation with stereo loudspeakers and two microphones (stereo microphones), two audio/voice channels are required to transmit the stereo signals. Another example of a multi-channel environment would be a conference room with two, three or four channel input/output. This type of applications is expected to be used on the Internet and in third generation cellular systems.
In a communication system, the available gross bitrate for a speech coder depends on the ability of the different links. In certain situations, for example high interference on a radio link or network overload on a fixed link, the available bitrate may go down. In a stereo communication situation this means either packet loss/erroneous frames or for a multi-mode coder a lower bitrate for both channels, which in both cases means lower quality for both channels.
Another problem is the deployment of stereo capable terminals. All audio communication terminals implement a mono-channel, for example adaptive multi-rate (AMR) speech coding/decoding, and the fall-back mode for a stereo terminal will be a mono-channel. In a multi-party stereo conference (for example a multicast session) one mono terminal will restrict the use of stereo coding and higher quality due to need of interoperability.
General principles for multi-channel linear predictive analysis-by-synthesis (LPAS) signal encoding/decoding are described in [3]. However, the described coder is not flexible enough to cope with the described problems.